1. Field of the Invention
The invention relates to semiconductor processing equipment and, more particularly, the invention relates to ceramic substrate supports.
2. Description of the Background Art
Susceptors are widely used to retain substrates, such as semiconductor wafers, within semiconductor wafer processing systems during processing. The susceptor is typically mounted to a pedestal. The pedestal is typically fabricated from a metal such as aluminum. The susceptor may be fabricated from laminated sheets of a polymer. However, for high temperature applications, the susceptor is typically fabricated from a ceramic material such as aluminum oxide or aluminum nitride. The susceptor typically contains various components which provide heating and/or cooling of the wafer as well as clamping (chucking) of the wafer to retain the wafer in a stationary position upon the pedestal surface. The pedestal may also include one or more electrodes for applying a bias voltage to the wafer. Such a bias voltage may be a direct current (DC) bias or a radio frequency (RF) bias.
Electrostatic susceptors (or chucks) retain a substrate by creating an electrostatic attractive force between the workpiece and the chuck. A voltage applied to one or more electrodes in the chuck so induces opposite polarity charges in the workpiece and electrodes. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece. These chucks find use in different types of wafer processing including etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) applications.
More specifically, an electrostatic chuck can be either xe2x80x9cmonopolarxe2x80x9d or xe2x80x9cbipolarxe2x80x9d. In a xe2x80x9cmonopolarxe2x80x9d electrostatic chuck, voltage is applied to the conductive pedestal relative to some internal chamber ground reference. Electrostatic force is established between the wafer and the chuck. In a xe2x80x9cbipolarxe2x80x9d electrostatic chuck, two electrodes are placed side-by-side (co-planar) to create the desired electric field. A positive voltage is applied to one electrode and a negative voltage is applied to another electrode. The opposite polarity voltages establish an electrostatic force that clamps wafer to the chuck.
The materials and processes used to process a semiconductor wafer are temperature sensitive. Should these materials be exposed to excessive temperature fluctuations due to poor heat transfer from the wafer during processing, performance of the wafer processing system may be compromised. To optimally transfer heat between the wafer and a chuck (or the chuck and the wafer), an electrostatic force is used to cause the greatest amount of wafer surface to physically contact a support surface of the chuck. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that interfere with optimal heat transfer.
To promote uniform heat transfer characteristics, an inert heat transfer gas (such as Helium or Argon) is introduced beneath the wafer to fill the interstitial spaces between the wafer and the chuck surface. This gas acts as a thermal conduction medium between the wafer and the chuck that has better heat transfer characteristics than the vacuum it replaces thereby promoting uniform heat conduction across the entire bottom surface of the wafer. Such a heat transfer gas is typically provided by channels drilled vertically through the body of the chuck from the wafer support surface to the bottom of the chuck. However, when the chuck is subject to a plasma, the heat transfer gas is prone to ignition thereby generating a plasma in the gas channels. The plasma in the gas channels sputters particles from the gas channel walls. The sputtered particles enter the processing chamber and contaminate the wafer.
Techniques, such as porous plugs and narrow diameter channels, have been tried to prevent plasma ignition in the orifices by attempting to increase the plasma free path. Electrons are neutralized (quenched) upon colliding with the ceramic walls of the pores or channels. Thus, the porous plugs and narrow, high aspect ratio orifices are designed to cause the electrons to quench on their walls before encountering a gaseous (He) atom, thereby preventing plasma formation within the gas channels.
Although these techniques do inhibit plasma ignition in the channels, there are considerable disadvantages. For example, the use of porous ceramic plugs complicates the fabrication of the chuck. Furthermore, porous ceramics tend to be chalky and produce particles that contaminate wafers during processing. The narrow orifices similarly increase the plasma free path and, therefore, inhibit plasma ignition in the channels. In the range of gas pressure and electric field normally encountered in the Helium ports, the diameter of the hole should be as small as possible. However, it is extremely difficult, time consuming and expensive to manufacture a ceramic chuck with small diameter holes bored entirely through the chuck body. Holes greater than 3 mm in diameter can be drilled in ceramics relatively easily using diamond drills. Holes 0.5 mm in diameter can be drilled through 3-15 mm of ceramic, at great expense, only by ultrasonic drilling methods. Unfortunately, the plasma free hole diameter is typically about 0.2 mm. Such holes can only be drilled through a thick plate by expensive laser drilling. Since the helium flow rate depends on the overall area of the holes, many small diameter holes (hundreds) are required to feed the helium fast enough.
Therefore, a need exists in the art for an easily fabricated ceramic electrostatic chuck having a backside gas distribution structure that inhibits plasma ignition in the gas delivery channels and a concomitant method of fabricating same.
The disadvantages associated with the prior art are overcome by the present invention of a susceptor having first and second ceramic layers. The first layer has a support surface, a bottom surface, and a plurality of ports therebetween. The second ceramic layer, is disposed beneath the first ceramic layer. A plenum, formed in the second layer, distributes of a heat transfer gas to the support surface. The first and second layers are stacked such that the bottom surface of the first layer forms a roof of the plenum. The first ceramic layer made thin to facilitate formation of multiple small diameter ports that communicate between the plenum and the support surface. The plenum is also made thin so that the small size of the ports and plenum inhibits plasma ignition inside the plenum. The plenum comprises, for example, a plurality of radially extending channels and at least one peripheral groove that communicates with said radially extending channels.
The structure of the susceptor is not limited to two ceramic layers. Any number of additional ceramic layers may be disposed below the second ceramic layer. The susceptor may also include one or more electrodes disposed within at least one of the ceramic layers. Any suitable number, pattern or type of electrode may be utilized. For example, the susceptor may include chucking, heating or bias electrodes.
The susceptor of the present invention may be fabricated by an inventive method. A first ceramic layer is formed to provide a support surface, a bottom surface, and a plurality of ports. A second ceramic layer is formed to provide a plenum. The second layer is disposed beneath the first layer such that the bottom surface of the first layer forms a roof for the plenum. The ports and plenum are aligned such that the ports in the first layer communicate with the plenum. The layers are cured to form a ceramic body by co-firing or hot pressing.
The reduced thickness of the first layer makes fabrication of multiple small diameter ports faster and less expensive. The small diameter ports in the susceptor of the present invention prevent plasma ignition inside the plenum. The ports can be distributed in any way that provides heat transfer gas to where it is needed, thus ensuring uniform cooling of a wafer supported by the susceptor.